Omni-directional antennas are often used for small cells such as so-called pico-cells and indoor coverage. Such antennas are also used at user terminals such as laptops and cell phones. The radiation pattern requirements will depend on which type of site and the propagation scenario that the antenna is intended for.
Most common in this context are vertically polarized omni-directional dipole antennas. When MIMO (Multiple Input Multiple Output) is introduced with several radio channels, multiple uncorrelated antennas are needed.
An uncorrelated sector antenna can be accomplished by using for example a patch-antenna with two orthogonal polarizations, vertical and horizontal. Orthogonal omni-directional antennas are considerably more difficult to accomplish, in particular an omni-directive, horizontally polarized wide band antenna. Most common is to use the horizontal distance between vertically polarized antennas to get uncorrelated radio channels.
Vertically polarized antennas must be well separated, in the order of several wavelengths, to achieve good multipath environment and therefore also good MIMO performance in all directions of the horizontal plane. However, large separation is not feasible when the antennas need to be integrated in small radio units and on low frequency bands. Half wave dipoles and micro strip patch antennas, for example, are relatively large, and a large separation between any two of these antennas becomes difficult to integrate in a small radio unit.
It is therefore a desire to provide a node in a wireless communication system that comprises an antenna arrangement that provides omni-directional coverage, an enhanced MIMO performance and that occupies a relatively small space.